We have consistently shown that synthetic organoselenium compounds (OSC) are superior cancer chemopreventative agents with less toxicity than selenite, naturally-occurring selenoamino acids, or their sulfur analogs. The proposed research focuses on our lead compound [1, 4- phenylenebis(methylene)selenocyanate, p-XSC] and will verify whether p-XSC or one of its metabolites is responsible for the chemoprevention of cancer; moreover it will seek to establish (a) plausible mechanism(s) for such an effect, especially during the post-initiation phase of carcinogenesis in the 7, 12-dimethylbenz(a)anthracene (DMBA) mammary tumor model in rats. Comparisons of our preliminary results with those obtained previously with benzyl selenocyanate (BSC), lead us to believe that glutathione conjugate (BSe-SG), a putative metabolite of BSC, has a higher chemopreventative index than the parent compound (BSC) in this DMBA mammary tumor model. In addition, BSe-SG is stable at pH ranges between 2 and 7.4 at 37 degrees Celsius. The efficacy of this conjugate in vivo indicates systemic absorption. However, it remains to be determined whether the intact conjugate or one of its metabolites is delivered to extrahepatic tissues. Our future studies, however, will focus on p-XSC, because we hypothesize that the glutathione conjugate (p-XSe-SG) is a primary metabolite of p-XSC, that will lead to the formation of an aryl selenol intermediate as the active chemopreventative principle. Support for the formation of an aryl selenol intermediate is based on the identification of tetraselenocyclophane (TSC) as a metabolite derived from p-XSC or p-XSe-SG. Thus, to test our hypothesis and to achieve the goals of this Project, in Aim 1, we will synthesize ample materials of p-XSC, p- XSe-SG, and TSC. To unequivocally confirm the presence of p-XSe-SG as a primary metabolite of p-XSC in vivo, we will conduct the synthesis and additional metabolism studies with the dual-labelled version of p-XSe-SG. In Aim 2, following determination of maximally tolerated doses, we will compare the efficacy of p-XSC, p-XSe-SG, and TSC in the initiation and post-initiation phases of carcinogenesis in the DMBA mammary tumor model in rats. The most effective compound will be further examined following its delayed administration (13 weeks after DMBA administration). Our results to date, together with literature data, appear to favor the hypothesis that p-XSC and/or one of its metabolites could inhibit oxidative damage, down-regulate cyclooxygenase (COX-2) expression, induce apoptosis and, consequently, inhibit carcinogenesis. Therefore, in Aim 3, we will examine the effect of those OSC that will emerge with significant chemopreventative activity from Aim 2 on markers that are especially important in the post-initiation phase of carcinogenesis, namely levels of lipid peroxidation (isoprostane), COX-2 expression, and apoptosis and on levels of 8-hydroxydeoxy-guanosine (8-OHdG). The results of this Project will provide insights into the feasibility of using p-XSC and/or one of its metabolites in future clinical trials on breast cancer chemoprevention and into the underlying mechanism(s) of action.